Corrosion control in sealed heat storage modules



March 17, 1970 I R ETAL CORROSION CONTROL IN SEALED HEAT STORAGE MODULESFiled Sept. 26, 1967 u E nwl'll"! United States Patent 3,501,261CORROSION CONTROL IN SEALED HEAT STORAGE MODULES Richard E. Rice andWillis Thompson Lawrence, Arlington, Mass., assignors, by mesneassignments, to Hookerv Chemical Corporation, Niagara Falls, N.Y., acorporation of New York Filed Sept. 26, 1967, Ser. No. 670,544

Int. Cl. C23f 11/06, 15/00; F28d 13/00 US. Cl. 212.5 9 Claims ABSTRACTOF THE DISCLOSURE BACKGROUND OF THE INVENTION In heat storage systems inwhich sensible and latent beats are stored at high temperatures ininorganic salt mixtures containing appreciable quantities of sodiumhydroxide or other alkali metal hydroxides, the heat storing medium ismost economically held in containers formed from ordinary mild steel,Such mixtures have thermal expansion coefficients substantially greaterthan that of steel. Consequently, it is necessary to provide anexpansion space in the container above the surface of the heat storagemedium.

Heretofore, this clearance space has been in communication with theambient air through a breather system through which air could enter andleave as the heat storage medium is cooled and heated, respectively.This has insured that no excessive pressures develop inside thecontainer, and that the heat storing salt mixture has accessperiodically to fresh air to maintain a substantially non-corrosivecondition in the container, according to a chemical process which isdescribed below. In order to prevent excessive heat loss to thesurroundings, the tubes connecting the clearance space to the outsideair must be small in diameter, and in order to prevent plugging bydeposits of sodium carbonate formed by reaction of the carbon dioxide inair with the sodium hydroxide, it is necessary to use chemicalabsorbents or special configurations of the breathing device. Copendingapplication Ser.

No. 556,230 filed by Richard E. Rice on Apr. 9, 1965, and US. 3,320,724,issued May 23, 1967, to Richard E. Rice, illustrate these embodiments.

In some applications of heat storage, it is highly desirable to avoidthe complications of such protective devices, by hermetically sealingthe container, thus preventing air from circulating through theclearance space. However, the alkali metal hydroxide compositions usedfor heat storing purposes cannot be operated in a sealed containerthroughout the desired temperature range which 'may extend to 900degrees Fahrenheit or higher, because of excessive gas pressures whichdevelop in the container, and because highly corrosive conditionsdevelop if such compositions are deprived of air.

Materials which have been most successfully used for heat storingpurposes in containers having breather systems comprise caustic soda,oxidizing agents such as sodium nitrate and/or sodium chromate, andcatalytic agents such as manganese dioxide. When heated above 3,501,261Patented Mar. 17, 1970 their fusion temperatures in steel containers,films of iron oxide form on the inner walls of the container. Thesefilms protect the steel from the excessively high rates of corrosion bythe caustic soda, which would otherwise occur. Concurrently with thefilm formation, the oxidizing components of the scale mixture arereduced, and to maintain a permanently non-corrosive condition such asystem requires continuous or periodic replenishment of the oxygen whichis consumed. If such a mixture is sealed in a steel container and heatedfor protracted periods of time above its melting point, the sodiumnitrate component, which in a typical case may be 8 percent of the totalweight of the heat storing material, may be reduced first to sodiumnitriate and subsequently to gaseous products which may include ammonia,water vapor, and hydrogen. This reaction can produce high pressuresinside the container and an excessively high rate of corrosion.

Prior art salt compositions containing even minor amounts of alkalimetal hydroxides have been noted for their tendency to corrode iron orsteel receptacles and equipment. For example, Beck in US. 2,211,047,issued Aug. 13, 1940, used alkali metal chlorides containing minoramounts such as 5-15 percent alkali metal hydroxides as heat transfermedia. He noted the excessive corrison of iron or steel equipmentcontacting his fused salt mixture and overcame the problem by additionof finely divided carbon into the heat transfer media. No mention ismade in this patent of the problems attending the prevention ofcorrosion in sealed heat storage units or the actual cause of thecorrosion problem.

Procedures have been recently developed for treating alkali metalhydroxides after which they can be hermetically sealed in ametalcontainer and operated safely without excessive corrosion of the innersurfaces of the container. In essence, if completely free of other saltsor impurities containing oxygen, and especially if completely free ofwater, caustic soda can under certain conditions be sealed in a steelcontainer without deleterious results.

Caustic soda has an extremely high afiinity for water and even whenheated considerably beyond its melting point of 605 degrees Fahrenheitwill retain quantities of water which are deleterious in sealed heatedcontainers. This water is very dilficult to eliminate, but it has beenremoved successfully by one or a combination of the followingprocedures.

One method consists in holding fused caustic soda for a period of timeat a temperature of 900 degrees Fahrenheit or above while in contactwith finely powdered metallic iron. This results in the elimination ofmoisture, and the saturation of the caustic soda with the by-products ofthe corrosion reaction. The principal chemical reactions involved arebelieved to be as follows:

Another method involves holding the caustic soda at a temperature ofapproximately 900 degrees Fahrenheit or higher and concurrently dryingit by maintaining an atmosphere of dry air in contact with the causticsoda.

A third method involves contacting the caustic soda under the conditionspresented in the preceding paragraph, with oxygen-free hydrogen ratherthan air.

As indicated in Equation I, one mole of iron powder reacts with one moleof water contained in the caustic soda. Taking into account molecularweights, one weight unit of water requires 3.1 weight units of iron forcomplete reaction. Therefore, a preferred ratio of iron powder to watercontained in the caustic soda is at least 3.1 to 1, respectively, byweight. In many practical cases, iron in excess of this ratio will beused to insure that water is comletely eliminated.

The caustic soda, having been treated as described above, is chargedinto a steel container leaving a clearance space at the top. Theclearance space may either be filled with hydrogen at atmospheric orsub-atmospheric pressure, or the air may be removed and the clearancespace left substantially evacuated. The container is then hermeticallysealed. The container is then heated relatively rapidly to a temperatureof at least 700 degrees Fahrenheit, at which temperature hydrogeninitially present in the clearance space, plus any additional hydrogenwhich is generated by reaction of the steel with caustic according toEquations I or II is rapidly eliminated by diffusion through the wallsof the container.

During subsequent operation as a heat storing element, the container isthermally cycled according to a schedule which maintains it for anappreciable fraction of time at a temperature above approximately 700degrees Fahrenheit, which insures that any additional hydrogen generatedinternally is eliminated by diffusion through the steel walls of thecontainer. Under these conditions the pressure in the clearance spacewill normally assume a value in the vicinity of 50 torr (millimeters ofmercury absolute).

It is an object of this invention to provide a method for reducing theinternal corrosion of hermetically sealed, steel containers for alkalimetal hydroxide heat storage compositions.

Also, it is an object of this invention to develop a steel heat storingapparatus which is adapted to house alkali metal hydroxide compositionswithout substantial corrosion internally.

Furthermore, it is an object of this invention to retard the corrosivechemical reaction occurring on the inside surfaces of a steel causticsoda storage container, by performing a reaction at the outside surfaceof the container which will retard the internal reaction by mass action.

Other objects and embodiments of this invention will become apparentfrom the following more detailed disclosure, examples and drawing.

BRIEF DESCRIPTION OF THE INVENTION We have now discovered that theinternal corrosion reaction in a hermetically sealed metal container,holding a heat storing medium containing an alkali metal hydroxide, maybe retarded by chemical reaction-s occurring on the external surfaces ofthe container. We have discovered that the rate of attack on theinternal surfaces of the sealed steel container, by the caustic soda,can be controlled by the composition of the gaseous atmosphere incontact with the outside surfaces of the container while at elevatedtemperature. For example, if the surrounding atmosphere contains anappreciable partial pressure of water vapor, the rate of corrosion onthe internal surfaces is reduced to or even the rate which occurs if thesurrounding atmosphere is completely chemically inert with respect tothe steel, or if the container is located in an evacuated space. Thus,it is within the scope of this invention to use the moisture content ofthe surrounding atmosphere, or the chemical composition of theatmosphere, or coatings which may be applied to the outside surfaces ofthe container, as means of controlling the internal corrosion rateduring the use of the container as a heat storing element.

A suitable coating material is one which has the ability to absorbmoisture from the air at temperatures near the lower end of thetemperature range through which the heat storage container is cycled innormal operation. At temperatures above approximately 700 degreesFahrenheit, where the internal corrosion rate would othenwise be high,this moisture is available to react with the steel according to theequation:

thus releasing atomic hydrogen to diffuse inward through the containerwall to oppose the production of hydrogen by the internal corrosionreaction and reduce the corrosion rate.

The temperature range through which a container of an alkali metalhydroxide heat storage composition may be cycled repeatedly for longperiods of time, in practice, varies with the thickness of the containerwall, the type of metal or alloy from which the container isconstructed, and with the specific heat storage material being used. Forexample, with thin steel containers housing an alkali metal hydroxidecomposition, the practical upper temperature limitation in operation isabout 900 degrees Fahrenheit. For other container alloys and other heatstorage materials the practical temperature limit is 1200 degreesFahrenheit and above.

Actually, by generating hydrogen on the external surface of thecontainer, the corrosion problem is not merely transferred to the outersurface, but the total corrosion of the container wall is reduced. Thecumulative corrosion of internal and external surfaces of a containerhousing an alkali metal hydroxide composition is reduced by a factor ofabout ten when a chemical reaction is performed at the outer surface ofthe container to produce hydrogen.

DETAILED DESCRIPTION OF THE INVENTION The complete heat storage modulecontemplated by this invention can best be exemplified and understood byreference to the accompanying drawing. It is understood that an externalcoating of material which is hygroscopic is not a critical element ofthis invention although it is exemplified in the drawing. Actually, anymethod of bringing water into reactive contact with the outside surfaceof the steel heat storage tank may be used to produce atomic hydrogen inproximity to the surface of the storage vessel.

The drawing illustrates a partial central section view of a heat storagemodule, comprising a steel shell 10 and two end closures 14, sealed at15, as by welding. The heat storage medium is an alkali metal hydroxidecomposition free of water which has been heated to 900 degreesFarhenheit in contact with iron powder, in a preferred embodiment of theinvention. Gas 13 is principally hydrogen, which under operatingconditions including high temperature operation, will pass through steelwall 10. A coating of a hygroscopic material 11 is placed upon theoutside surface of the steel tank wall 10 to remove moisture from theatmosphere during periods of low operation temperature.

Example 1 A small steel container (hereinafter called a capsule) waspartially filled with caustic soda and hermetically sealed. This capsulewas placed in a glass and fused quartz vacuum system which contained ameans for heating and maintaining the capsule at the desiredtemperature. The apparatus was also equipped with vacuum pumps, a valveby which the portion of the apparatus containing the capsule can beisolated from the vacuum pump, and a pressure measuring device formeasuring the pressure in the isolated portion of the system. The onlygaseous product produced by the drying and corrosion reactions washydrogen. By maintaining the capsule at a temperature in the 700 800degrees Fahrenheit range, the hydrogen diffused through the steel wallsof the capsule. When a steady state was reached, the rate of dilfusionwas the same as the rate at which hydrogen was produced. This rate wasdetermined by closing the vacuum valve to isolate the portion of thesystem holding the capsule and measuring the rate of rise of pressurewhich resulted from eflusion of hydrogen from the capsule. The rate ofthe corrosion reactions were then determined from the constants of thesystem and the rate of pressure rise. Under the conditions of thisexperiment, the outside surface of the capsule was contacted by hydrogenonly, at a pressure in the range of about 5 10- torr. When the test wasfinally terminated, the corrosion rates were confirmed by mechanicalmeasurements made on the walls of the container itself.

The corrosion rates determined by this method were found to be between0.012 and 0.040 inch per year. This corrosion rate is approximately 50times as great as the corrosion rates occurring when the walls of thevessel are exposed to an atmosphere containing water vapor.

Example 2 A sealed cylindrical container having a capacity ofapproximately 700 grams of caustic soda was prepared. Inside thecontainer two sheet steel coupons approximately 1" x 2" were positionedso that one was totally submerged in the molten material and the otherso that most of its surface was exposed in the clearance space above themolten caustic soda. The container was heated at 900 degrees Fahrenheitwith its exterior surfaces exposed to air containing water vapor at apartial pressure of 3-15 torr. The corrosion rate occurring on the innersurfaces of the container and on the samples was determined at intervalsby noting the changes in weight which occurred and by measuring thechange in the thickness of the metal sheets. The corrosion ratedetermined by this method was between 0.0002 to 0.0007 inch per year.

The diiference in corrosion rates between the preceding comparativeexamples was attributed to the different atmospheres or externalenvironments of the two types of containers. The container used for theweighed coupon measurements was surrounded by the ambient air whichnormally contains an appreciable partial pressure of water vapor,whereas the capsules used in the hydrogen effusion test were completelyblanketed by the emitted hydrogen which contained no appreciable watervapor.

It is clear from re-examination of Equations I and II that an increasein the pressure of hydrogen in such a system will tend to shift theequilibrium toward the left in both cases, by mass action. An increaseof hydrogen pressure will decrease the corrosion rate on the internalsurface of a sealed heat storage vessel containing an alkali metalhydroxide composition. In the range 700-900 degrees Fahrenheit, atomichydrogen diffuses very readily through the steel walls of the container,and the effective hydrogen pressure at the corrodiug surfaces, i.e., theinternal surface of the container, can therefore be influenced by acondition which produces atomic hydrogen at the external surfaces.

Atomic hydrogen is in equilibrium with molecular hydrogen. Thesolubility of atomic hydrogen in steel and the possible formation ofmolecular hydrogen within pockets or the interstices of the steel wallof a container may directly influence the diifusion of hydrogen throughthe steel.

It is believed that only atomic hydrogen diffuses through steel underthese conditions, whereas molecular hydrogen does not pass through thesteel. The hydrogen gas in the atmosphere surrounding the container isalmost entirely molecular, and at practical pressures it would have aninappreciable influence on the internal corrosion rate.

Water vapor in the surrounding gas, however, can react with the iron atthe outside surfaces of the container as is illustrated by Equation III,supra. The atomic hydrogen thus produced at the external surface canexert a pressure opposing that of the hydrogen produced on the internalsurface by the corrosion reaction and thus influence the rate ofcorrosion. Likewise, the pressure of hydrogen resulting from thecorrosion reaction tends to reduce the rate of the oxidation reaction,thus each reaction opposes the other.

are suflicient to have a profound influence on the corrosion rate of theinternal surfaces of a steel heat storage container. This method ofcorrosion control can also be applied through the use of coatingsapplied to the outside surface of the container. For example, a coatinghaving the ability to absorb moisture from the air at a temperaturebelow approximately 700 degrees Fahrenheit may be used to increase themoisture concentration at the surface of the container in thetemperature range above 700 degrees Fahrenheit. Examples of suitablecoating materials include such things as silica gel, alumina andzeolites.

The extension of this idea to other metals is obvious. And in itsbroadest aspect, this invention embraces the concept that anycombination of chemical and metal whose reaction produces atomichydrogen can be controlled from the other side of the metal by anotherreaction which also produces atomic hydrogen.

Example 3 A heat storage container such as that described in theaccompanying drawing, was filled with caustic soda which had beenprepared for use in a sealed container for heat storage purposes by thefollowing procedure.

A commercial grade of flaked caustic soda manu factured by the diaphragmcell process was heated to a temperature of 900 degrees Fahrenheit atwhich the material was molten in the steel melting pot.

Water was removed by holding the fused material at 900 degreesFahrenheit for 24 hours while in contact with dry air. One percent ofminus 325 mesh electrolytic iron powder was then added to remove anyremaining water. The prepared caustic soda was then charged into acylindrical container like that described in FIGURE 1. The top closurewas welded into place, and the air in the clearance space removed with avacuum pump and the container was completely sealed by welding.

The heat storing container was then tested by thermal cycling at therate of four cycles per day between a top temperature of approximately800 degree Fahrenheit and a bottom temperature of 200 degreesFahrenheit, for eight months, with the outside surface of the containerexposed to the ambient air which contained appreciable Water vapor. Thiscaused the caustic soda to melt and solidify during each cycle. Duringthis cycling, no excessive pressure developed in the clearance space, nodeleterious corrosion occurred on the internal surfaces, and nodistortion of the container resulted from the forces exerted bycontraction and swelling of the solidified caustic soda.

Examples 45 To further investigate the function of water on the outsidesurface of a sealed module containing an alkali metal hydroxidecomposition, the following experiments were conducted.

An apparatus similar to that disclosed by Bloom et al., Journal of theElectrochemical Society, vol. 104, No. 5, pp. 264-269, was constructed.In essence, a vacuum system was constructed with a manometer, coldfinger containing water and a capsule compartment. Valves were providedwhich would isolate the cold finger containing Water from the capsuleand to isolate the system from the vacuum pump.

The capsule was heated to a temperature of 900 degrees Fahrenheit withthe valve to the cold finger closed and valves to the manometer andvacuum pump open. The rate of hydrogen generation at any instant wasmeasured by closing the valves to the vacuum pump and manometer andmeasuring the pressure change per unit time on the manometer. Otherequivalent pressure measuring devices may be substituted for amanometer. With the valve to the cold finger closed, only a small amountof hydrogen is present in the system. Thus, the rate of pressure changeis indicative of the corrosion rate in a vacuum and is directly relatedto the corrosion rate when the volume of the system, the internalsurface area of the capsule and the equation governing the corrosionprocess are known.

To determine the corrosion rate with water vapor on the outside of thesealed capsule containing an alkali metal hydroxide composition, thecold finger is used. The water in the cold finger is brought to aconstant temperature while the valve isolating the water from the restof the apparatus is closed and while the valve to the vacuum pump andmanometer are open. Then the valves to the vacuum pump and manometer areclosed and the valve to the water in the cold finger is opened. Thisintroduces water vapor into the system with a partial pressure equal tothe saturation pressure of water at the constant temperature of thewater in the cold finger as regulated by a constant temperature bath.The hydrogen generated by corrosion on the inside and outside of thecapsule wall passes into the system causing the total pressure to riseabove the partial pressure of the water vapor. The difference betweenthe partial pressure of water vapor and the total pressure at anyinstant is the pressure of hydrogen in the system. The rate of change ofhydrogen pressure may be used to calculate the corrosion rate. In thiscase, however, it is not known what portion of hydrogen is generated bythe two dilferent reactions. Therefore, only an overall indication ofcorrosion rate may be obtained. It is apparent, however, that thepresence of Water vapor on the outside of the capsule of alkali metalhydroxide composition significantly reduces the total corrosion rate.The reduction of the corrosion rate is proportional to some positivepower of the water vapor pressure. The following data presents theresults obtained in three experiments. These experiments arerepresentative of the reduced corrosion rate observed when water vaporcontacted the outside wall of a capsule containing an alkali metalhydroxide composition.

Having disclosed this invention, it will become apparent to thoseskilled in the art that obvious modifications and variations may be madewhich do not depart from the true spirit of this contribution. Thepreceding examples are presented herein for the purpose of illustratingspecific embodiments of this invention rather than to limit its truescope. It is to be understood that although reference is made throughoutthis specification to atomic hydrogen and its diffusion through steel,this is a theoretical consideration only, while the inventive conceptinvolved is not so restricted. When reference is made to hydrogen oratomic hydrogen, it is intended that the form of hydrogen which diffusesthrough steel or other metal alloys is embraced.

What is claimed is:

1. A process for retarding the corrosion of the metal of a hermeticallysealed container at from about atmospheric to sub-atmospheric pressurehousing an alkali metal hydroxide composition that reacts with saidmetal to generate hydrogen which comprises producing hydrogen at theexternal surface of the'container by chemi cal reaction at a temperaturesufficient to effect diffusion of hydrogen through the container wall.

2. The process of claim 1 in which the metal of said sealed container isiron.

3. The process of claim. 1 in which the temperature at which hydrogen isproduced at the outside surface of the container is above the meltingpoint of said alkali metal hydroxide composition.

4. A process for reducing internal corrosion within a hermeticallysealed steel container at from about atmospheric to sub-atmosphericpressure, said container being substantially filled with an alkali metalhydroxide composition, which comprises producing hydrogen at the outsidesurface of the container at a temperature above the melting point of thealkali metal hydroxide composition.

5. The process of claim 4 in which hydrogen and atomic hydrogen aregenerated at the outside surface of the container by the reaction ofatmospheric water vapor with the iron of the steel container.

6. An apparatus comprising a hermetically sealed steel containersubstantially filled with an alkali metal hydroxide composition, saidcontainer being coated .with a hygroscopic material upon the exteriorsurface corresponding to the interior surface which contacts said alkalimetal hydroxide composition.

7. The apparatus of claim 6 in which said coating material is a memberselected from the group consisting of silica gel, alumina and a zeolite.

8. A process for reducing internal corrosion within a hermeticallysealed steel container at from about atmospheric to sub-atmosphericpressure, said container being substantially filled with an alkali metalhydroxide composition, which comprises producing hydrogen at the outsidesurface of the container at a temperature above the melting point of thealkali metal hydroxide composition by the reaction of atmospheric watervapor with the iron of the steel container, said atmospheric Water vaporbeing held in close proximity to the outside surface of the steelcontainer by a hygroscopic coated on said outside surface.

9. The process of claim 8 in which the hygroscopic coating material is amember selected from the group consisting of silica gel, alumina and azeolite.

References Cited UNITED STATES PATENTS 2,211,047 8/1940 Beck 252712,453,471 11/1948 SWitZeI' et a1. 20684 XR 2,791,074 5/ 1957 Woodman206-84 XR 2,924,274 2/ 1960 Richardson.

3,028,955 4/1962 Shorkey et al. 206-84 3,081,241 3/1963 Smith 2l-2.7 XR3,299,945 1/1967 Rice et al l04 XR 3,320,724 5/1967 Rice 55269 3,382,9195/1968 Rice 165-105 MORRIS O. WOLK, Primary Examiner B. S. RICHMAN,Assistant Examiner US. Cl. X.R.

"( 53 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.350 261 Dated March U. 1970 lnventofls) Richgrd E, Rice and Hi llis ThawIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 33, delete "steel," and insert steel. Column 2, line l l,delete "nitriate" and insert nitrite Column 6, line #0, detete "degree"and insert degrees Column 8, Claim 8, line H, after "hygroscopic" insertmaterial SIGNED AN SEALED JUL 2 81970 Anew Edward M. Fletcher, 11-.:gfiiff- 55mm,

A g officer can or Paton"

